The recovery of mineral and coal fines presents considerable difficulties with the result that large losses of valuable resources are incurred in many operations. The separation of water contaminated with oil or other organic liquids is also difficult. Froth flotation is currently the most widely used method for fine particle processing but the cost of the flotation cells which are used is high. In a paper entitled "Study of Coal Flotation Practice--RD & D Requirement" published by BHP Central Research Laboratories in 1988, it was estimated that the base cell structure of mechanical flotation cells, including the rotor and stator accounts for almost 80% of the cost of a bank of cells, with the drive motors representing a further 6.5% of the cost. Accordingly, a significant reduction in the residence time (which dictates cell volume) would lead to a dramatic reduction in the capital costs of the cell and support structure.
Flotation is typically a process in which product particles in suspension are separated from reject particles on the basis of differences in their surface chemistry characteristics. The component to be floated is usually naturally hydrophobic or rendered hydrophobic by addition of suitable reagents. When air is introduced into the slurry system, the hydrophobic particles adhere to the air bubbles forming particle/bubble aggregates which rise to the surface of the flotation cell where they are removed as froth. Being hydrophilic, the remaining constituents of the slurry stay in suspension and are discharged as waste material which is generally referred to as tailings.
It has been identified that in the design of a flotation cell, four functions need to be accomplished, namely:
(i) transportation of hydrophobic material to a contacting zone, PA1 (ii) generation of bubbles, PA1 (iii) promotion of hydrophobic material-bubble collision in the contacting zone, and PA1 (iv) separation of hydrophobic material/bubble aggregates. In most flotation systems, particularly those which utilise mechanical agitation by impellers, the optimum levels of these four functions are not coincident. Additionally, the zone in which separation is performed is typically large due to slow transfer rates with the result that large process tanks are required. PA1 (a) flow inducing means for providing a flow of the feed; PA1 (b) gas introduction means for introducing a gas and forming bubbles of the gas in the flow; PA1 (c) motionless mixing means for dispersing the gas bubbles in the flow and causing contact between the hydrophobic material and the gas bubbles; and PA1 (d) centrifugal separation means in fluid communication with the motionless mixing means for separating, from the flow as froth, hydrophobic material/gas bubble aggregates formed by contact between the hydrophobic material and the gas bubbles. PA1 (a) providing a flow of the feed; PA1 (b) introducing a gas into the flow whereby the flow contains bubbles of the gas; PA1 (c) subjecting the flow containing gas bubbles to motionless mixing to disperse the gas bubbles in the flow and cause contact between the hydrophobic material and the gas bubbles, whereby hydrophobic material/gas bubble aggregates are formed in the flow; and PA1 (d) introducing the aggregates containing flow into centrifugal separation means whereby aggregates are separated from the flow as froth. PA1 (a) a generally vertically orientated cylindrical vessel; PA1 (b) inlet means located in a wall of the vessel for introducing the flow into the vessel whereby a downwardly spiralling flow is formed within the vessel with consequential separation of aggregates as a rising froth and waste material; PA1 (c) first outlet means located below the inlet means for removal of waste material from the vessel; PA1 (d) second outlet means located above the inlet means for removal of froth from the vessel; and PA1 (e) a generally horizontally orientated partition which spans the walls of the vessel and which is located between the inlet means and the second outlet means dividing the vessel into an upper portion and a lower portion, the partition having means for permitting the upwardly moving passage of froth through the partition and means for permitting the downwardly moving passage of waste material through the partition.
Flotation has traditionally been carried out in mechanical flotation cells which are basically continuously stirred tank reactors in series. In these cells, particles are kept in suspension by impellers which also generate the bubbles and promote particle-bubble collision and attachment thus fulfilling the last three of the functions mentioned above. The turbulence which promotes particle bubble attachment also causes disruption of particle/bubble aggregates and mechanical entrainment of hydrophilic particles into the resultant froth. Additionally, the separation of the negatively buoyant hydrophobic particle/air bubble aggregates from the pulp containing the hydrophilic particles is driven by the force of gravity which makes it necessary for the slurry being processed to have a residence time within the flotation cell of the order of 3-5 minutes for coal and significantly longer for mineral applications. The design features of these cells therefore limit the overall efficiency of the flotation treatment of fine particles.
In recent years, the utilisation of countercurrent flotation columns to increase the efficiency of the flotation process has received significant attention. In these systems, air bubbles are generated continuously at the bottom of a column using spargers. The bubbles rise through the downwardly flowing slurry which is fed to the column at a position about two-thirds of the column height, resulting in bubble-particle collision and attachment. The grade of the product is further improved by washing the froth with water to minimise entrainment of hydrophilic particles. Several variations of the column have been designed, e.g. the Leeds Column, the Davcra cell, the Filblast column and the Microcel. The Microcel uses centrifugal pumps and inline mixers to generate micro-bubbles. While the grade is significantly improved by use of flotation columns, the yield or recovery may suffer and the capacity is relatively low due to the need for long residence times.
A more recent Australian development is referred to as the Jameson cell which is described in U.S. Pat. No. 4,938,865, AU 76108/91 and AU 83980/91. In this system, air is added to the slurry which is flowing down a long vertical tube. A flow restriction such as an orifice plate, promotes the formation of air bubbles which are intimately mixed with the particles before entering a tank or column where separation of froth and pulp occurs. The froth is removed from the top of the tank in the usual manner.
A number of new processes are being developed based on the concept of flotation in a centrifugal field.
An example of this type of technique is that referred to as the Air Sparged Hydrocyclone (ASH) which is described in U.S. Pat. No. 4,279,743, U.S. Pat. No. 4,397,741, U.S. Pat. No. 4,399,027 and U.S. Pat. No. 4,744,890. In this system, a prime feature is the use of a micro-porous material through which air is injected into a cylindrical cyclone. The porous material forms an inner tube in the cylindrical outer shell. Although the system has high, capacity, the cost of the micro-porous material makes the unit expensive as well as being subject to significant maintenance and wear problems. Additionally, differences in hydrostatic head leads to uneven dispersion of air bubbles.
Another system is that referred to as the Centrifloat Rapid Flotation System which is described in Brake IR, Graham JN, Madden RG and Drummond RB (1993) "Centrifloat Pilot Scale Trial at Goonyella Coal Preparation Plant", in Davies J. J. (ed), Proc. Sixth Australian Coal Preparation Conference, Paper G1, pages 364-400. This system uses a cyclone inlet to generate a swirl of the feed slurry which is introduced at the bottom of an open-top vessel. Air is injected upstream of the feed inlet via a cylindrical wall of a microporous material similar to that used in ASH. The froth migrates to the centre of the vessel and is collected at the top of a catchment basin. This dispersion of the air is unlikely to be efficient, reducing the probability of particle capture, particularly in situations where significant hydrostatic pressure variations exist.
A system described in AU 65432/90 also uses centrifugal action to enhance flotation. In this system the centrifugal action is mechanically generated. The cost of the system is adversely affected by the capital cost of the mechanism generating the centrifugal action, the power requirements and the maintenance of the moving parts.
Another system is that referred to as the Flotation Cyclone which is described in WO91/19572. This system utilises a flotation cylinder having a porous wall similar to ASH with the cylinder partitioned into upper and lower ends with a tangential inlet located adjacent the upper end.
A further system is described in U.S. Pat. No. 4,971,685 in which the flotation process has been partitioned into two discrete operations, namely bubble-particle contact and froth-pulp separation. A hydrocyclone with two entry ports, one for feed slurry and one for bubbles, acts as a bubble-particle contactor before discharging via a single exit at the apex of the cyclone into a shallow froth separation unit where the mineral laden froth is removed from the top. In this system the bubbles are generated externally and the centrifugal action is not utilised to enhance separation but merely for particle-bubble contact.
A further system is that referred to as the Fastflot process which uses high velocity clean liquid jets as air and energy carriers with the flotation separation taking place in a curved bottom tank.